The use of digital, as opposed to analog signals, for television broadcasts and the transmission of other types of video and audio signals has been proposed as a way of allowing improved picture quality and more efficient use of spectral bandwidth over that currently possible using analog NTSC television signals.
The International Standards Organization has set a standard for video data compression for generating a compressed digital data stream that is expected to be used for digital television and with digital video tape recorders (VTR's). This standard is referred to as the ISO MPEG (International Standards Organization--Moving Picture Experts Group) ("MPEG") standard. One version of the MPEG standard, MPEG-2, is described in the International Standards Organization--Moving Picture Experts Group, Recommendation H.222.0, ISO/IEC 13818-1"Information Technology--Generic Coding Of Moving Pictures and Associated Audio" dated Nov. 13, 1994, ("the ISO 13818-1 International Standard") hereby expressly incorporated by reference.
The MPEG-2 Systems layer, which is the syntax and semantics for the transport packetization and multiplexing of an MPEG-2 bitstream, also referred to as a data stream, is described in detail in the ISO 13818-1 International Standard. The MPEG-2 systems layer carries within it a program clock reference (PCR) of the system clock for each program in a multiplex of programs being transmitted. The PCRs are used, e.g., at decoding time, e.g., by a phase locked loop circuit to synchronize a decoder system clock to the system clock of an encoder originally used to generate the bitstream being decoded. The MPEG-2 systems layer also includes a presentation time stamp (PTS) and may include a decoding time stamp (DTS). The PTS and DTS values are samples of the PCR value divided by 300. The PTS and DTS values indicate the tine at which a particular frame represented by data in the bitstream is to be displayed or decoded, respectively. The PCR, PTS and DTS values included in a bitstream allow a decoder to correctly decode the bitstream thereafter display the video and/or audio of, e.g., a television program represented by the bitstream.
As part of the MPEG-2 standard, constraints are placed on the timing information which must be included in an MPEG-2 bitstream. In particular, MPEG-2 requires that the duration between two subsequent encodings of PCR signals must not exceed 100 msec in time, where the time is measured in terms of the time required to transfer the data, and the duration between two subsequent encodings of the PTSs must not exceed 700 msec., where the time is measured in terms of display time.
Thus, the MPEG standard provides various constraints, e.g, signal timing requirements, that are intended to insure that a bitstream can be decoded and displayed properly.
It is expected that digital VTR's which record MPEG encoded video bitstreams will have special areas on a tape in which trick play data, i.e., data intended to be displayed during one or more modes of VTR trick play operation such as fast forward and reverse, is recorded. The trick play data may be video data extracted at recording time from a received bitstream, e.g., from data intended to be displayed during periods of normal VTR playback operation. During trick playback operation trick play data will be read from a tape by the VTR and output to a decoder for trick playback decoding.
For the trick play data recorded on the tape to be fully MPEG-2 compliant, e.g., for it to form a syntactically legal MPEG transport bitstream when read, it must be encoded prior to recording assuming a certain playback speed.
If a VTR uses trick play data recorded assuming a certain playback speed, e.g., assuming 4.times. fast forward playback operation, for display during an additional mode of trick playback operation, e.g., at 2.times. fast forward, the recorded PCR, PTS and DTS values will not be correct for the additional mode of VTR operation.
In such a situation, i.e., where data was recorded assuming one playback speed, and the data is read back at another playback speed, there is a need to correct the PCR, PTS and DTS values read from the tape to produce an MPEG-2 compliant bitstream which can be decoded and displayed, e.g., by am MPEG compatible decoder and display device such as a television set attached to a VTR.
Accordingly, there is a need for methods of recording data in a manner that facilitates its subsequent reading and decoding into an MPEG compliant bitstream, and for correcting the timing information contained in a bitstream, e.g., a trick play data bitstream, so that it conforms to MPEG standards during, e.g., trick playback operation.